This application relates to optical data processing and communication devices and systems, and more specifically, to techniques and systems for polarization-division multiplexing in digital optical communication devices and systems.
An optical carrier at a carrier wavelength can be used as an optical communication channel to provide a large bandwidth in signal transmission due to its inherent large carrier frequency. Hence, the optical carrier transmitted through an optical link, which may be implemented in either free space or in an optical waveguide (e.g., fiber), can be used in high-speed and broadband communication systems. However, the actual useful bandwidth of a single optical carrier may be limited by a number of factors, including the material dispersion and optical nonlinearities of the optical fiber, and the operating speeds of electronic components associated with the optical channel.
One way to further increase data capacity in an optical link is to simultaneously transmit optical carriers of different wavelengths so that different channels of data can be carried by different carriers and sent over the optical fiber link at the same time. This technique is known as xe2x80x9cwavelength-division multiplexingxe2x80x9d (xe2x80x9cWDMxe2x80x9d). To further increase the transmission capacity, dense WDM (xe2x80x9cDWDMxe2x80x9d) techniques have been developed to increase the number of multiplexed wavelengths in a single optical link by reducing the wavelength spacing between two adjacent wavelengths to a few nanometers or even in the sub-nanometer range.
The techniques and systems disclosed in this application include polarization-division multiplexing (xe2x80x9cPDMxe2x80x9d) to use different states of polarization in a single optical carrier at a transmitting terminal, to multiplex different channels of data to produce a PDM signal for transmission. The respective receiving terminal decodes the received PDM signal, without demultiplexing, to separate the different states of polarization, to extract the different channels of data. Hence, the data capacity of a single optical carrier can be increased. Such a PDM signal at one carrier wavelength may be multiplexed with one or more other PDM signals at different carrier wavelengths in a WDM or DWDM system.
In one implementation, a PDM transmitter is configured to combine first and second optical beams of two different polarizations modulated to respectively carry first and second channels of binary data to produce a polarization multiplexed signal. This signal has unequal power contributions from the two polarizations to have four possible distinct power levels to represent the first and second channels of binary data. A PDM receiver may be designed to receive the polarization multiplexed signal generated by the PDM transmitter and to produce two output signals respectively representing the first and second channels of binary data according to a power level of the polarization multiplexed signal with respect to said four distinct power levels, without recovering said different polarizations.